Mayweed Chamomile (Anthemis cotula) is a dicot plant in the asteraceae family. A single amino acid substitution from Proline 197 to Leucine has led to resistance to ALS inhibitors as indicated in the table below.

Mayweed Chamomile

Chemical Family

Example Herbicide

Resistance Level

Imidazolinones

Imazethapyr

Intermediate Resistance < 10 fold

Pyrimidinylthiobenzoates

Bispyribac-Na

Not Determined

Sulfonylureas

Chlorsulfuron

Resistant > 10 fold

Triazolopyrimidines

Chloransulam-methyl

Intermediate Resistance < 10 fold

Sulfonylaminocarbonyltriazolinone

Flucarbazone-Na

Not Determined

NOTE

Different Pro197 substitutions were present within populations, and therefore specific cross resistance patterns conferred by specific substitutions are confounded.

Mayweed chamomile seeds were collected from six different fields across the Pacific Northwest. All populations (each collection site was considered a population) were suspected to have some level of acetolactate synthase (ALS) resistance. Greenhouse and laboratory studies were conducted to determine if these populations were resistant to three different classes of ALS inhibitors: sulfonylureas (SU), imidazolinones (IMI), and triazolopyrimidines (TP). A whole-plant dose-response and in vitro ALS activity studies confirmed cross-resistance to thifensulfuron+tribenuron/chlorsulfuron (SU), imazethapyr (IMI), and cloransulam (TP); however, resistance varied by herbicide class and population. Two ALS isoforms of the ALS gene (ALS1 and ALS2) were identified in mayweed chamomile; however, only mutations in ALS1 were responsible for resistance. No mutations were found in ALS2. Sequence analysis of the partial ALS gene identified four point mutations at position 197 (Pro197 to Leu, Gln, Thr, or Ser) in the resistant populations. This study demonstrates genotypic variation associated with cross-resistance to ALS inhibitors within and between populations..